PRINCIPLES OF MACHINES 



465 



where there is pressure or slow motion; light lubricants 

 are used for high-speed machines, in which pressure is not 

 great. The following terms are used in the description 

 of lubricants: 



Specific gravity: Weight of the lubricant as compared 

 with pure water. 



Viscosity: Number of seconds required for a given quantity 

 to pass drop by drop through a small opening at a fixed 

 temperature. 



Flash point: Temperature at which the oil will begin to 

 give off a gas which will burn. 



Fire point: Temperature at which the oil will continue to 

 support a flame. (The difference between the flash point 

 and the fire point is usually not many degrees.) 



Chill point, or cold point: Temperature at which the oil 

 will cloud and cease to flow. 



Rolling surface. A cylindrical-shaped body rolling over 

 a surface has less friction than a plane sliding over another 

 plane. Where the space 

 will permit and expense 

 justify, the insertion be- 

 tween the two surfaces of 

 a series of very hard metal 

 balls or rollers substitutes 

 rolling for sliding friction. 

 This is the principle ap- 

 plied when four wheels 

 are placed under the load 

 on the wagon. There are 

 several types of this form 

 of a non-friction device 

 (Fig. 354). Some are 

 made up of spheres; 

 others, of cylindrical or taper rollers, known as ball and 

 roller bearings. These bearings not only lessen the friction, 

 but they assist in spreading and holding the lubricant. 



Fig. 355. 



After International Harvester Co. 



A shaft supported by roller 

 bearings 



